The colder and snowier it gets, Canadian scientists have discovered, the more penis length matters for mating success.

A study of mammals from around the world has found that the species with the best-endowed males live in polar regions, rather than in more equatorial climes.

That's because a longer penis increases the chances of being the successful father when females mate with a succession of different males, say researchers Steven Ferguson and Serge Larivière in the current issue of Oikos, a Nordic scientific journal.

"It's a competition among the different sperm, but the penis is the instrument that carries this out," says Ferguson, a marine biologist at the federal government's Freshwater Institute in Winnipeg.

A long penis means the male's sperm gets closer to the female's egg than other sperm, Ferguson explains.

The head of the longer penis can also damage the sperm left by competing males that were less well endowed.

Ferguson says penis length isn't as important for species in which a male gathers a harem by battling other males.

In that case, brute strength has decided the reproductive winner before the mating takes place.

But in harsh climates &#8212; where mating opportunities are so infrequent that females have a go with any available male &#8212; penis length becomes a crucial factor in deciding which males get to be involved in fatherhood.

The two biologists didn't investigate the actual length of the penises of living, breathing animals &#8212; which might involve some risky measurements &#8212; but instead studied the length of a bone, called the baculum, found inside the penis of many animals.

This can be safely measured in museum collections.

Biologists don't understand why some animals sport a baculum, while others don't. The baculum is very small in the entire cat family, vestigial in primates and non-existent in humans, says Larivière.

"It makes so little sense to have a bone there because fractures are common, and if the penis bone breaks, you're out of business for that mating season &#8212; and maybe longer."

Larivière says the bone adds support to the penis but is not essential for copulation.

Instead, "baculum size serves as an indictor of male quality.

"If a female encounters a dozen males at the same time, she can evaluate them as potential fathers and make a choice.

"But if you're a female wolverine, who only encounters one male at a time, you've got to have another method of selecting the best father."

It's a case of mating first and asking questions afterward, Larivière wrote in an e-mail.

Yet scientists can only speculate about how those questions are asked and how female physiology manages to favour fertilization by sperm from a better-endowed male after first mating with one whose equipment doesn't measure up.

One way might be a hormone-release triggered through stimulation by the longer baculum. The hormones could then bring on an abortion of fetuses that are in waiting.

Support for this theory comes from breeding studies undertaken on captive minks. A female mink will abort fetuses created by mating with one male if a bigger male comes along later and mates.

Other animals can delay implantation of fertilized eggs in the uterus, providing another way to favour one prospective father over another.

In their quest to uncover why bacula vary so much in size, the biologists compiled a league table of average penis-bone lengths for 122 kinds of carnivores who have a baculum.

The lengths were rated relative to body size, so larger animals like bears needed to have a proportionally bigger penis to be in the running.

Using an arsenal of sophisticated statistical methods, the scientists found positive correlations between the bone's length and factors like snowfall and the latitude where the species lived, but not with other environmental or biological factors.

In the Arctic, for example, the bacula of walruses can reach 55 to 60 centimetres, twice as long as the penis bone in elephant seals in warmer California waters.

The seals weigh much more than the walruses, but they rule over harems.

In contrast, Ferguson says, walruses are spread so thinly across the Arctic that males almost never fight and females usually mate with any and every male that shows up.

"Her choice about who will be the father takes place after the copulation and the longer baculum gives that male's sperm a better chance of succeeding."

Other researchers have suggested that the shorter penis of the elephant seals could be an evolutionary adaptation designed to reduce the risk of fracture, since the seals mate on land while walruses mate in the water.

But Ferguson points out that penis bones can break during mating in the water as well.

The long and short of the two cases, he concludes, lies in the different demands from female and male promiscuity.

The current research was carried out using funds from the Natural Sciences and Engineering Research Council, one of three federal granting agencies.

But the two scientists have been studying bacula and other evolutionary aspects of reproduction for a decade, ever since both were doctoral students at the University of Saskatchewan.

"We got started out of pure curiosity," says Larivière, who serves as general manager of the Quebec federation for trappers and trap-line managers.

"I had seen penis bones in animal remains out on trap lines when I was a 10-year-old. It was a puzzle I'd had since then,"

Larivière says the research he and Ferguson have conducted into life-history traits has been limited by the paucity of studies into bacula.

About some aspects, they can only speculate.

For instance, skunks generally have a small penis bone. But some species of skunks from northern regions appear to be setting up harems as their preferred reproductive strategy.

Gold Member

The baculum itself is interesting. Only five orders of mammals have them; primates, rodents, insectivores, carnivores, and chiropterans (bats). A good acronym to remember is PRICC. A theory is that two molecular clades -- euarchontoglires (primates, rodents, and a few other rodent-like taxa) and laurasiatheria (ungulates, cetaceans, carnivores, insectivores, and chiropterans) -- are sister taxa due to this homology. Humans are one of the few primates without a baculum, so it can clearly disappear.

Gold Member

I've been wondering whether a large penis might show anything about having good genes generally. It's not necessary to the argument above - a long one might be favoured on simple sexual selection; but if it also indicated good genes or good health it would encourage such a selection means. I'm wondering whether a large penis requires a better blood circulation to get it up, and therefore a large erect penis is an indicator that the individual has good circulatory health and genes?